Mass spectrometry is used widely in many fields, such as physics, chemistry, biology, medial science, pharmaceutics, agriculture, and engineering. Analysis of atoms, molecules and organic compounds by mass spectrometry first calls for ionization.
Conventionally, ionization has been accomplished mainly by use of an electron-impact type ion source. The impingement of electrons in this type of ion source, however, imparts many complex mass spectra to a specimen of organic compound as a result of fragmentation. Then it often becomes difficult to obtain a characteristic spectrum (especially for molecular ions) necessary for identification and structural analysis.
A solution proposed is field ionization (hereinafter called the FI method). This method employs an anode that consists of a metal wire on the surface of which conductive microneedles are grown and an opposite cathode disposed several millimeters away from the anode. A strong electric field is formed on the surface of the metal wire by applying a voltage of over 10 kv between the anode and the cathode.
On supplying a gasified organic compound specimen having a high vapor pressure, the metal surface absorbs electrons and causes ionization.
Because it emits ions, the metal wire having the conductive microneedles on its surface is called an ion emitter or emitter.
Another method proposed for ionizing a specimen with a low vapor pressure is ionization by field desorption (hereinafter called the FD method). According to this method, a liquefied or suspended specimen is put on a metal wire on which conductive microneedles are grown as in the case of the above-described FI method (which is also called an emitter).
The emitter is placed in an ion source as an anode, spaced approximately 2 mm away from an opposite cathode. An electric field of approximately 10.sup.8 v/cm is formed in the vicinity of the specimen on the conductive microneedles by applying a voltage of over 10 kv between the anode and the cathode. By the tunneling effect, the electrons in the specimen passes through the potential barrier distorted by the strong electric field to the metal wire. Then the remaining positive ions are taken away from the emitter by the electric field around the opposite cathode and enters the mass spectrometer to perform analysis.
The mass spectra thus produced by the FI and FD ionizing methods are suited for the determination of the molecular weight of a compound because they have strong molecular ion peaks and few peaks resulting from fragmentation.
As evident from the above description of the ionization mechanisms of the FI and FD methods, their ionization efficiency depends on the quality of the emitter.
A good emitter should have the following three properties:
(1) High ionization efficiency.
(2) Ability to hold much specimen.
(3) Adequate strength.
Most emitters have been prepared by growing graphitelike conductive microneedles on a tungsten wire, which has a diameter of approximately 10 .mu.m and is heated to approximately 1200.degree. C., by applying a high voltage of 10 to 14 kv to the wire in a stream of benzonitrile (C.sub.6 H.sub.5 CN) under reduced pressure. This type of emitter will be called a carbon emitter hereinafter.
In manufacturing, however, the tungsten wire needs careful pretreatment and such a long time as 5.about.10 hours is required in order to grow the microneedle crystals. Further, the 10 .mu.m diameter tungsten wire with low mechanical strength easily breaks during use because of electrical shocks due to discharge and contact in putting a specimen thereon.
The object of this invention is to provide a semiconductor ion emitter which can be manufactured easily in a short time, has an adequate mechanical strength, can hold much specimen thereon, and assures high-efficiency ionization.
The semiconductor ion emitter according to this invention achieves this object by employing an electrode that comprises a number of semiconductor whiskers standing on the conductive peripheral surface of a wire having a diameter of about 60 .mu.m.
Further, a process for manufacturing the semiconductor ion emitter according to this invention comprises the steps of evaporating gold onto the peripheral surface of a wire on which whiskers of a semiconductor are to be grown in a vacuum atmosphere, preheating the wire, supplying a gas containing the semiconductor at a regulated pressure so as to control the growth of the whiskers on the base, and heating the wire at a regulated temperature.
An apparatus for manufacturing the semiconductor ion emitter comprises a vacuum vessel to enclose in a vacuum atmosphere the wire on which whiskers of a semiconductor are to be grown, means for heating the wire in the vacuum vessel, the heating means having a temperature control function, and means for supplying a gas containing a semiconductor into the vacuum vessel, the gas supplying means communicating with the vacuum vessel through a control valve.